This invention relates to machine vision measurement systems and in particular to the calibration of such systems using a calibration artifact in conjunction with calibration software.
Machine vision measurements systems are used to inspect parts and components after production to insure they and their associated features meet specified tolerances. For example, a stamped stock piece with one or more bores may be placed on a stage under a video camera and viewed on a monitor in real time. The monitor is connected to a computer with measurement software operating thereon and, by using a mouse and/or a keyboard, the diameter of each bore can be accurately measured to evaluate whether the stamped piece was correctly manufactured for quality assurance purposes.
Machine vision measurement systems are capable of different magnification levels to accommodate parts of different sizes and to more accurately measure and view features of different sizes on any one part.
The measurement systems themselves are typically calibrated before they are installed at a user""s facility and then routinely during use to insure the measurements taken are accurate. Typically, a calibration artifact is used to calibrate machine vision measurement systems. The artifact includes a number of different size circles and squares etched on one surface of a glass substrate. Adjacent each circle and square is its size. For example, there may be nine circles ranging in diameter from 0.01 mm to 5.0 mm.
The artifact is placed on the measuring stage and the calibration routine of the measurement software initiated. The technician then chooses a magnification level, and positions the calibration artifact such that the largest circle which fits on the monitor screen is chosen for accuracy reasons. The technician then reads the diameter of that circle off the artifact and enters it as an input to the calibration routine. The calibration routine then automatically correlates the size of the pixels on the monitor screen to the size of the entered known diameter so that, for the magnification level chosen, the number of pixels which make up any given distance or area is known.
The primary problem with this prior art calibration method is that the above process must be repeated for each magnification level. That is, for each magnification level, the calibration artifact must be repositioned such that the largest circle now occupies the viewing screen, and the diameter of that circle entered.
In addition, when using video to measure objects, it is very important that the lighting be correct. It is possible to introduce distortion of the image by over saturating the image. When such over saturation occurs, the dark parts of the image appears to shrink and the light parts seem to grow. One term for this is blooming. Excess light from an edge in the image encroaches on dark pixels and they start to register as white. Extreme cases of overexposure are easily detected by an operator, but when subpixeling for accuracy, subtle effects of blooming can seriously effect measurements and lead to errors.
Moreover, when the smallest size circles are in use (at high magnification), the pixel resolution is small (a 10 micron spot may fill the frame). The effect of manufacturing the artifact off by a micron is much larger than measuring off by a pixel. On the other hand, if the pixels are large (low magnification) the effect of measuring off by a pixel is much more than manufacturing off by a micron. Thus, one problem associated with prior art artifacts is a problem called xe2x80x9cover or under etchingxe2x80x9d. The pattern is laid down quite repeatability, but then the chrome is etched away. If the artifact is left in the etch too long, the chrome circles will be smaller on the outer dimension, and larger on the inner dimension. This effect is very similar to blooming.
Accordingly, the calibration artifact and the calibration method associated with the prior art results in a tedious and time consuming calibration process subject to errors if the technician incorrectly enters data, if blooming occurs, or if the artifact is not manufactured to strict tolerance levels and under or over etching occurs.
It is therefore an object of this invention to provide a new, easier to use calibration artifact for calibrating a machine vision measurement system.
It is a further object of this invention to provide such an artifact which renders the calibration procedure less complicated, less time consuming, and less prone to errors.
It is a further object of this invention to provide such a calibration artifact which allows the implementation of a more automatic calibration process.
It is a further object of this invention to provide a method of calibrating a machine vision measurement system which overcomes the problems associated with prior art methods.
It is a further object of this invention to eliminate the requirement that the technician must reposition the calibration artifact each time a different magnification level is chosen when calibrating a machine vision measurement system.
It is a further object of this invention to eliminate the need for the technician to input the diameter of each circle on the artifact when the magnification level changes.
It is a further object of this invention to eliminate the effects of blooming and over or under etching.
This invention results from the realization that the problems of prior art machine vision measurement system calibration methods can be overcome by a unique calibration artifact with concentric rings of different sizes and which does not have to be repositioned when different magnification levels are chosen, which eliminates the need for the technician to input the diameter of each circle when the magnification level changes, and which eliminates the effect of blooming.
The size of each ring and the change in diameter between each adjacent pair of rings is stored in the calibration software. One aspect of the invention is that the change in the size between each adjacent pair of rings is different. In this way, when the calibration software measures how many pixels occupy the two largest rings on the monitor screen, the calibration software can calculate the ratio of pixels occupying the two rings, and from that ratio, automatically establish the actual (calibrated) size of either ring. Once this information is known, the effective size of the pixels at that magnification level is set for future measurements and, by repeating the above process at each magnification level, the machine vision measurement system is thus properly calibrated without the need to ever reposition the artifact, or enter size information.
This invention features a calibration artifact for calibrating a machine vision measurement system. The calibration artifact comprises a substrate and a plurality of concentric rings on one surface of the substrate, each ring of a different pre-defined size. In the preferred embodiment, the change in the size of any two adjacent rings is different than the change in size of any other two adjacent rings.
The method of calibrating a machine vision measurement system of this invention includes placing a calibration artifact including a series of concentric rings under a camera of the machine vision measurement system; choosing a magnification level; measuring the size of a first largest ring in pixels; measuring the size of a second largest ring in pixels; comparing the sizes; and determining, from the comparison, the actual diameter of one of the rings. Each ring is of a pre-determined different size and the change in the size of any two adjacent rings is different than the change in size between any other two adjacent rings. The method may further include determining a first average of the measured size of the first largest ring in pixels and the measured size of the second largest ring in pixels. The method may further include measuring the size of a third largest ring in pixels and determining a second average of the measured size of the third largest ring in pixels and the measured size of the second largest ring in pixels. Comparing may include using the first and second averages.
Typically, each ring is of a pre-determined different size and the change in the size of any two adjacent rings is different than the change in size between any other two adjacent rings.
The calibration artifact preferably includes a substrate and a plurality of concentric shapes (e.g., rings) on one surface of the substrate. Each shape has a different pre-defined size and the change between the size of any two adjacent shapes is different than the change in size between any other two adjacent shapes.
This invention also features a calibration system including a calibration artifact and a software algorithm. The artifact includes a substrate, and a plurality of concentric rings on one surface of the substrate. Each ring is of a different pre-defined size and the change between the size of any two adjacent rings is different than the change between the size of any other two adjacent rings. The software algorithm includes a database containing the size of each ring and data reflecting the change in size between each pair of adjacent rings.
The method of calibrating a machine vision measurement device comprises placing a calibration artifact including at least one ring with inner and outer edges under a camera of the machine measurement system; choosing a magnification level; measuring the size of the outer edge of the ring in pixels; measuring the size of the inner edge of the ring in pixels; and averaging the measured size of the outer edge of the ring and the inner edge of the ring.